Re: [Xen-devel] Xen 4.2 Release Plan / TODO

["Ren, Yongjie" <yongjie.ren@xxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of Matt Wilson
> Sent: Wednesday, June 27, 2012 6:57 AM
> To: Konrad Rzeszutek Wilk
> Cc: Ian Campbell; xen-devel
> Subject: Re: [Xen-devel] Xen 4.2 Release Plan / TODO
> 
> On Tue, Jun 26, 2012 at 02:09:06PM -0700, Konrad Rzeszutek Wilk wrote:
> > On Tue, Jun 26, 2012 at 01:31:51PM -0700, Matt Wilson wrote:
> > > On Tue, Jun 26, 2012 at 01:39:42AM -0700, Ian Campbell wrote:
> > > >     * [BUG] vcpu-set doesn't take effect on guest, reported by Intel:
> > > >       http://bugzilla.xen.org/bugzilla/show_bug.cgi?id=1822
> > >
> > > This may be a problem with the guest, rather than a problem with Xen
> > > or the toolstack. I tested on a domU running a 3.2.20-based Linux
> > > kernel. The domU kernel didn't automatically enable the vCPUs after
> > > calling "xl vcpu-set" to increase the allocation.
> > > [...]
> > >
> > > I've added this as a comment to the bugzilla.
> >
> > That is a generic bug - its a race in the generic code where
> > the cpuX directory is created; then udev kicks off and tries to
> > echo 1 > cpuX/online (but online hasn't been created yet); the
> cpu-hotplug
> > code creates 'online' directory.
> >
> > I've asked Greg KH about it, and here is his take:
> > https://lkml.org/lkml/2012/4/30/198
> >
> > But I never got to prep a patch. If somebody gets to do it before me,
> > I owe them a beer!
> 
> My problem was that I didn't even have a udev rule to auto-online
> hotplugged CPUs. Everything works as expected after adding the rule
> suggested on the wiki:
> http://wiki.xen.org/wiki/Paravirt_Linux_CPU_Hotplug
> 
> I'm thinking that the bug report is just this well hashed out
> problem. For historical reference, here's the thread discussing the
> behavior change in pv-ops kernels:
>   http://lists.xen.org/archives/html/xen-devel/2010-05/msg00516.html
> 
Hi Matt, thanks for the useful status.
According to my testing on xen-unstable(#25517) with linux3.4.3 dom0 and RHEL6.2
hvm guest, 'xl vcpu-set' can increase the number of the vCPU for guest. But it
can't decrease the number of vCPU. After trying to decrease the number of vCPU,
the number of the onlined CPU in /sys/devices/system/cpu/ or /proc/cpuinfo is 
not
changed.

But I can do "echo 0 > /sys/devices/system/cpu/cpu3/online" to offline the CPU3
in the guest even if I didn't use any command line like 'xl vcpu-set'.

Also, I can do "echo 1 > /sys/devices/LNXSYSTM:00/LNXSYBUS:00/LNXCPU:03/eject"
to remove CPU3 in the guest.

Note that 'CPU3' can be any CPU except for CPU0.

Also add a comment in the bugzilla.


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